Abstract The inferior colliculus (IC) is a major site for integration of virtually all ascending and descending auditory pathways, and many fundamental characteristics of IC neurons are profoundly shaped by their inhibitory inputs. A significant source of GABAergic inhibition to the IC originates from a brainstem circuit formed by the medial nucleus of the trapezoid body (MNTB) and the nearby superior paraolivary nucleus (SPON). The long-term goal of this research program is to clarify the functional role of the MNTB/SPON circuit in hearing. Our central hypothesis is that the MNTB/SPON circuit extracts and encodes auditory stimulus discontinuities, and that the output of this circuit contributes to temporal processing in the IC. The objectives of this application are i) to determine how MNTB and SPON neurons integrate their synaptic inputs to generate the output of this circuit, and ii) to examine the impact of the MNTB/SPON circuit's efferent projection on response properties of IC neurons. Three Specific Aims, each focusing on a specific step along the pathway from the MNTB to the IC, are proposed. Specific Aim 1 will directly examine the influence exerted by the MNTB/SPON circuit on sound processing in the midbrain, focusing primarily on the ability of IC neurons to detect gaps in tones, and to synchronize their responses to sinusoidally amplitude modulated tones. A dual recording configuration will be used to reversibly inactivate SPON neurons, while simultaneously recording from their postsynaptic targets in the IC. In Aims 2 and 3, we will perform in-vivo recording studies to clarify the roles of inhibition and excitation in shaping the response properties of SPON and MNTB neurons to stimulus discontinuities. A multi-step, sequential protocol will be employed to systematically block specific neurotransmitter systems in order to reveal their individual contributions to single-unit responses as a whole. Comprehensive characterizations of each recorded cell will be performed. The acuity with which stimulus discontinuities are detected between closely placed sounds or components of sounds plays an important role in speech perception, and deficits in temporal processing are related to language impairment. Therefore, a better understanding of the function of the MNTB/SPON circuit in animal models will advance our knowledge of the neural basis for human speech comprehension.